Coalton is an efficient, statically typed functional programming language that supercharges Common Lisp.

Coalton can be written in files:

(in-package #:coalton-user)

(named-readtables:in-readtable coalton:coalton)

(coalton-toplevel
  ;; Define Coalton `Symbol`s as Lisp `cl:keyword`s.
  (repr :native cl:keyword)
  (define-type Symbol)

  ;; Bind a Lisp function into Coalton.
  (declare sym (String -> Symbol))
  (define (sym s)
    "Create a new symbol named `s`."
    (lisp Symbol (s)
      (cl:intern s "KEYWORD")))

  ;; Define equality of `Symbol` types using CL's `eq`.
  (define-instance (Eq Symbol)
    (define (== a b)
      (lisp Boolean (a b)
        (cl:eq a b))))

  ;; Define a new parametric algebraic data type for simple
  ;; mathematical expressions.
  (define-type (Expr :t)
    "A symbolic expression of basic arithmetic."
    (EConst :t)
    (EVar   Symbol)
    (E+     (Expr :t) (Expr :t))
    (E*     (Expr :t) (Expr :t)))

  ;; The classic `diff` function, in Coalton.
  (declare diff (Num :t => Symbol -> Expr :t -> Expr :t))
  (define (diff x f)
    "Compute the derivative of `f` with respect to `x`."
    (match f
      ((EConst _)                       ; c' = 0
       (EConst 0))
      ((EVar s)                         ; x' = 1
       (if (== s x) (EConst 1) (EConst 0)))
      ((E+ a b)                         ; (a+b)' = a' + b'
       (E+ (diff x a) (diff x b)))
      ((E* a b)                         ; (ab)' = a'b + ab'
       (E+ (E* (diff x a) b)
           (E* a          (diff x b))))))

  ;; We can use `t` just fine since Coalton doesn't import `cl:t`.
  (define t (sym "t"))

  (declare dt (Num :t => Expr :t -> Expr :t))
  (define dt
    "The time derivative operator."
    (diff t)))

And at the REPL:

CL-USER> (in-package #:coalton-user)
COALTON-USER> (coalton-toplevel
                (define (square x) (E* x x)))
; No value
COALTON-USER> (coalton (dt (E+ (square (EVar t)) (EConst 1))))
#.(E+ #.(E+ #.(E* #.(ECONST 1) #.(EVAR |t|))
            #.(E* #.(EVAR |t|) #.(ECONST 1)))
      #.(ECONST 0))

Type errors are discovered at compile-time, and errors are printed beautifully without sacrificing Common Lisp's interactive debugging facilities.

COALTON-USER> (coalton (dt (E+ (EConst 1/2) (EConst 0.5))))
error: Type mismatch
  --> <unknown>:1:30
   |
 1 |  (coalton (dt (E+ (EConst 1/2) (EConst 0.5))))
   |                                ^^^^^^^^^^^^ Expected type '(EXPR FRACTION)' but got type '(EXPR DOUBLE-FLOAT)'
   [Condition of type COALTON-IMPL/TYPECHECKER/BASE:TC-ERROR]

Restarts:
 0: [RETRY] Retry REPL evaluation request.
 1: [*ABORT] Return to top level.
 2: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {10013A8003}>)

Coalton has not reached "1.0" yet. This means that, from time to time, you may have a substandard user experience. While we try to be ANSI-conforming, Coalton is currently only tested on SBCL 2.2.x, Allegro CL 10.1. and Clozure CL 1.21.*.

Getting Started

Prepare: Install SBCL (on macOS with Homebrew: brew install sbcl). Install Quicklisp by following instructions here. (The step command involving gpg is not needed.) After installing Quicklisp, you should have a quicklisp folder which will make installing Coalton easier.

Install: Clone this repository into a place your Lisp can see (e.g., ~/quicklisp/local-projects/). Coalton is not yet on Quicklisp.

Use: Either run (ql:quickload :coalton), or add #:coalton to your ASD's :depends-on list. Quicklisp will automatically download all of Coalton's dependencies.

Test: Compile the tests with (ql:quickload :coalton/tests), then run the tests with (asdf:test-system :coalton).

Learn: We recommend starting with the Intro to Coalton document, and then taking a peek in the examples directory. It may also be helpful to check out the introductory blog post.

What's Here?

This repository contains the source code to the Coalton compiler, and the standard library.

It also contains a few example programs, such as:

• Some simple pedagogical programs,
• An implementation of Jones's Typing Haskell in Haskell, and
• An implementation of a simple Quil parser using parser combinators.

Lastly and importantly, we maintain a collection of documentation about Coalton in the docs directory, including a standard library reference guide.

Get Involved

Want to ask a question about Coalton, propose a feature, or share a cool program you wrote? Try posting in the GitHub Discussions page!

We welcome contributions of all forms, especially as we stabilize toward a 1.0 release. We would be grateful to receive:

• bug reports (filed as issues),
• bug fixes and typo corrections (filed as pull requests),
• small example programs, and
• user experience troubles.